The transplantation of spleen cells from old NZB/Bl mice with renal disease induced both the structural and the functional changes of membranous glomerulonephritis in young NZB/Bl mice within a few weeks and well in advance of its usual spontaneous occurrence. The development of hypergammaglobulinemia and lymphoid cell hyperplasia in the young mice indicated that immunologically competent cells, derived from either the transplant or the recipient, proliferated during this process. These experiments, together with other findings, provide further support for the view that membranous glomerulonephritis in NZB/Bl mice is produced by immunological, and probably autoimmune, mechanisms and that the renal disease is apparently almost wholly unrelated to the hemolytic process.